Mercado Fuencarral
The shopping areas in Madrid seemed quite simple to categorise. All the high street shops like Zara, Mango, H&M are huddled around Sol, the Old Town. Most upscale and designer brands are in Salamanca for the business people and the pijos (posh) who hang out there. Finally, the cool and trendy shops are stacked in Chueca, the gay district (becoming more and more mixed) especially on calle Fuencarral.
Best place in Madrid I found for T-shirts and streetwear? Hands-down, the Mercado Fuencarral, a grungy, two-floor mall (akin to those in Ximen) in Chueca.

Image courtesy of Florencio Sanchez